The intracellular mechanisms that control growth cone navigation to appropriate target cells are required to build networks of functional neural connections during nervous system development and regeneration. Our long-standing interest in the cytoskeletal signaling machinery downstream of multiple guidance receptors has led us to a series of conserved microtubule-associated proteins that appear to mediate different types of guidance behavior. Analysis of the effector protein CLASP has defined a pathway from the repellent factor Slit and its Roundabout receptors, to the Abelson tyrosine kinase, to CLASP, as a means of impeding microtubule and leading edge advance. How Abl acts to coordinate microfilament and microtubule dynamics, and control the activity of CLASP or its associated partners is unknown. Preliminary data indicates that several proteins directly or indirectly linked to CLASP are required for accurate midline repulsion, whereas other microtubule effector proteins appear to play distinct roles in early axon guidance decisions. We will use a combination of genetics, cell biology and biochemistry to determine each protein's function in axon guidance and to dissect the signaling mechanisms that regulate the key effector activities.